Wellbore equipment sometimes depends on injection molding processes during manufacture. For example, a progressive cavity pump includes a stator tube with an elastomer material on the interior surface thereof. During operation, a rotor disposed at least in part inside the stator tube operates in conjunction with the stator to pump fluids down a wellbore and transfer pressure or drilling materials downhole. Injection molding processes are typically utilized to cast the elastomer material inside the stator tube.
Injection molding usually requires a mold core that occupies a volume inside the stator tube so the occupied volume does not fill with elastomer material during the injection process. With respect to a progressive cavity pump, this volume may be part of the space where the rotor will operate. Operating conditions, such as elastomer pressure, elastomer temperature, uneven flow of elastomer into a helical space, mold core thermal expansion, and/or mold core weight and length, may cause the mold core to displace downward throughout the injection molding process, a phenomenon sometimes referred to as “sagging.” Sagging is also affected by the tendency of elastomer to fill the bottom portions of an annular passageway before the upper portions (when injecting horizontally), thereby causing the elastomer to travel at irregular rates along the outer circumference of the mold core. If the mold core sags, it may cause operational inefficiencies for the completed pump, based on irregularities in clearance between the rotor and the stator and/or other factors.
There are some known attempts to remedy the effects of mold core sag throughout the injection molding process. One example includes drilling pins through the stator tube to hold the mold core in place at various points along the mold core's axial length. However, this pin method still allows sag because there are unsupported points along the mold core. The pin method also increases operational complexity and costs by adding a step to fill the pin holes in the stator tube after injection molding.